Many products required by today's markets are made by molding parts of plastics and other materials. One of the most costly and time-consuming steps in the manufacture of these products is the preparation of molds. A most common mold design generally consists of working inserts of a core and cavity, which contain a working surface with critical requirements of dimensional tolerance and surface finish. Also, inserts contain parting lines and a mold frame, which provides structural integrity to the mold element. In order to prepare molds having an active surface that provides accurate reproduction of the products with every manufacturing cycle, the active mold surface must have a hard detailed finish which in most processes requires complex machining. Such molds must be long lasting so that their cost can be amortized over as large number of manufacturing cycles as possible. All of the problems of mold production are exaggerated when the product has intricate well-defined surfaces to replicate.
In today's world of computer aided design and modeling almost every step in the manufacturing process has been upgraded to accommodate high-speed operations. This presents a real dilemma for toolmakers, as the extended periods required for mold manufacture remains a serious problem. Mold making therefore remains the only low speed operation in the manufacturing process. For example, a typical tooling operation may be optimized to about 14-18 weeks preparation time.
As will be seen, improvements in tooling and surface finish thereof represents one of the purposes of this invention, which is to provide a high-speed method of mold making with a savings in time of about 20 to 30 percent. In addition, mold lifetime is an important factor for many tooling applications, particularly for die-casting or glass forming processes. The present invention is intended to improve significantly such tool life due to the use of new effective tooling materials and new techniques to improve adhesion of the active layer of the mold to the mold substrate surface. Furthermore, in many tooling applications, the function of heat removal is critical as it defines the productivity of the manufacturing process and quality of some cooling-sensitive materials like glass, nylon etc. The present invention, therefore, has the additional object to offer solutions to such issues, which leads to improvement to injection mold heat transfer.
In order to create fine detailed mold elements, and to minimize finish machining, powdered metal casting has become a popular process. The use of powdered metals to make a tools is well established as shown in U.S. Pat. No. 4,327,156; 4,431,449 and 4,455,354. In the methods of the prior art, as disclosed by these patents, a mold or tool element is constructed of a skeleton made of a first metallic powder preform having microscopic interconnected pores filled with a second infiltrating metal. Tool elements of this type are strong, reliable, and can be used to form many complex and detailed shapes. The choice of metal used depends on the application of the tool and can include almost an infinite number of combinations.
A process of manufacturing mold elements is disclosed in the above referenced '354 patent, and is worthy of detailed review. This method consists of constructing a master model of a product to be replicated and forming a mold of rubber or other flexible material therefrom. Appropriate metal powders are mixed together with a heat fugitive thermoplastic binder and then the powder-binder mixture is applied into a rubber mold, and allowed to chill at room temperature. Following low temperature heating of the powdered-binder the preform burns off binder and forms a porous skeleton to be filled with an infiltrating alloy. At a next higher temperature heating, the porous preform is filled with molten infiltrating alloy under capillary forces. At both stages of heating, the powder preform is surrounded and supported with another powder, which does not wet and infiltrate with the molten infiltrating alloy. Therefore an infiltration front stops at a border between wettable and non-wettable powders. Due to very fine particles used in this powder system, a smooth surface with any type of detail can be obtained. Such surface requires a polishing only to be accorded the requirements of a tool.
The above mentioned infiltration method provides the ability to compose materials with different useful properties. For instance, applying carbide powders allows preparation of a high wear resistance tool. The use of corrosion-resistance-infiltrating alloy allows combination of a high wear and corrosion resistance etc. Therefore, the '354 disclosure and related patents completely accord to the requirements of toolmakers from the point of view of surface quality and specific properties of tooling materials. However, these methods do not satisfy a third critical requirement: dimensional accuracy of infiltrated articles. This requirement is so critical that its omission overcomes the above mentioned advantages. The dimensional accuracy problem has therefore stimulated efforts to increase tolerances and dimensional predictability of infiltrated products. Nevertheless, in spite of a number of attempts, there has not yet been achieved a successful result. In fact, a dimensional threshold of acceptable tolerance at +/-0.1% from a total size has yet to be reported.
Because the overcoming of this dimensional threshold is one of the important objects of the present invention, a detailed analysis of sources of inaccuracy, by way of background, is reviewed below.
First, there are several technological steps at the fabrication of infiltrated articles, which are reflected in '354 and similar patents with some variations: making a rubber mold, preparation of a powder preform, burning off a temporary binder and infiltration followed by solidification and cooling. Each link of this technological chain brings its effect on total inaccuracy.
The loss of accuracy starts from a dimensional non-stability of the rubber mold during its fabrication. Then, thermal instability of the rubber mold and non-uniformity of shrinkage of the powder-binder mixture also generates an inaccurate replication of the powdered tool preform. Furthermore, the evacuation of binder by heating the powdered preform in a non-wettable powder medium, according to '354 patent, presents an additional inaccuracy due to shrinkage of the molten binder. An additional loss of accuracy takes place during the infiltration process as the supporting action of an inert powder is not sufficient to prevent possible slumping of the powder preform under its weight. Finally, one further source of inaccuracy according to '354 patent is a non-uniformity of shrinkage of composite material during the solidification of an infiltrating alloy as well as distortions during cooling of the infiltrated article.
A total accumulated error therefore requires the necessity of finish machining of infiltrated articles, which reduces the value of this manufacturing method. Because tool inserts according to '354 patent are made as a solid block with drilled cooling channels, there are no specific solutions offered to improve the heat exchange within an insert. Due to an insufficient accuracy of methods of the '354 and similar patents, such methods have therefore failed to develop any broad applications in the tooling industry in spite of potential usefulness and numerous attempts.
Further to the above, it is noted that at the step of rubber mold fabrication, dimensional errors are caused by shrinkage of the self-curing rubber. That is, as a rule, a silicon rubber, with 0.2% average shrinkage is used for making the rubber molds. After a mold is released from an initial rigid model, it suffers from additional dimensional changes, which are produced by relief and balancing shrinkage stresses within its rubber body. The produced dimensional change is not uniform or predictable due to the complicated shapes and broad variety of possible configurations of tools.
For instance, a mold can comprise local lots with different character of shrinkage progress; in some lots the model may reduce the shrinkage, but in other lots this shrinkage may be free. This situation creates a shrinkage non-uniformity, which is increased by shrinkage stresses after releasing off a model.
In addition, at the step of powder preform fabrication a molded thermoplastic mixture heats the rubber mold, or it may be heated previously to obtain a perfect surface of the powder castings. The heating of rubber is accompanied by a thermal expansion up to 1.5% when heated to 80.degree. C. An irregular configuration of rubber mold therefore generates an irregular expansion for different mold volumes. As a result, the prior configuration of the mold surface can be distorted, especially at lots within rubber walls. Therefore, the direct or non-direct heating of a rubber mold generates an additional source of inaccuracy of a final tool. The following cooling of the powder preform induces shrinkage of both the powder preform and rubber mold. Because the shrinkages are different, stresses cause a distortion of both structures, while the rubber distortion is reversible and the powder preform distorts non-reversibly.
At the step of debinding a powder preform is subjected to stepwise heating with the purpose to burn out the temporary binder. The debinding includes the sequence processes of melting thermoplastic binder, capillary flow into a porous supporting non-wettable powder and vaporization at the final stage of debinding. The melting of binder is accomplished with significant expansion, which is characterized as 14% (vol.) for the most commonly used paraffin-based binders. Therefore, the melting of binder causes expansion of the powder mixture and a powder preform on the whole. The front of binder melting moves within a powder preform with temperatures about even to the melting point of a binder. Such a front of melting crosses a powder preform, creating an alternative expansion of different volumes of the preform.
Expansion of exterior volumes is attenuated by absorption of molten binder into a non-wettable powder, but for internal layers this effect is less significant. The redistribution of molten binder between the powder preform and non-wettable powder causes a piecemeal wane of binder contents from 100% of space between particles of preform to zero at the end of debinding. As binder moves out, it develops compressive capillary forces inside of the preform, as well as making the powder structure more compact along with changing dimensions of the powder body. The maximal value of these forces takes place with a content of binder at 60-70% from its initial content. As it follows, a powder body during debinding is crossed with two waves of dimensional change: the wave of expansion caused by melting of binder and the second wave of contraction caused by the capillary-compression effect of molten binder. Such phenomena produces an additional loss of accuracy in the final tool.
At the step of infiltration physical effects also occur, which are similar to the debinding process, but follow at reverse order. At the first stage of infiltration the spreading of molten alloy at the surface of particles occurs. At this moment the space between particles is not filled completely and powerful capillary-compression forces take place. These forces produce movement, and regrouping, and make the packing of particles tighter. At the second stage of infiltration capillary forces involve additional portions of molten alloy to completely fill the space between the particles. The complete filling turns the three-phase system "liquid--gas (or vacuum)--solid" to a two-phase system "liquid--solid". It also eliminates capillary forces and changes the powder-molten alloy system to the condition of a suspension, which keeps the specific properties of fluids and can flow under its own gravity. The possibility of infiltrated powder to move is forced additionally by increasing the weight of powder preform in comparison to the weight of infiltrating alloy. Because non-wettable powder is not bonded, its supporting function is limited. These circumstances may create a significant modification of preform configuration in the direction of gravity in addition to prior accumulated errors.
According to the teachings of '354 and similar patents, an infiltrating alloy is disposed immediately on the back surface of the powder preform. Such technique entails significant interaction between molten alloy and solid powder. As a result of this interaction an undesirable change of chemical compositions of both components can occur. Because the majority of components have a definite solubility, such interaction could lead to a change in the phase composition of the infiltrating alloy and its temperature of solidification. This can create the phenomena of isothermal solidification of the infiltrating alloy that makes complete infiltration impossible. Therefore the disclosed technique limits significantly the number of possible combinations of powders and infiltrants.
Furthermore, the disclosed solidification of infiltrating alloy and cooling of the composite body generate non-uniform reducing dimensions of a tool due to a gradient of temperature and the specific configuration of the tool. After transition of the composite material to the elastic state, the stresses from deceleration of shrinkage create a distortion of the tool body, which is the final source of dimensional errors noted above.
The '354 patent does not use a thermoplastic binder for forming a powder preform, therefore the thermoplastic binder sources of dimensional errors are not acting directly present therein. However, the problems associated with infiltration with a molten alloy, and the following solidification and cooling are still present. Additionally, the use of a rigid ceramic mandrel causes additional dimensional and surface finish problems. It is a significant dimensional accuracy problem due to the shrinkage during ceramic processing, especially for drying and fairing ceramic articles. Therefore, existing methods of ceramic part fabrication do not allow for preparation high accuracy mandrels pursuant to the teachings of the '354 patent. Also, employing machinable ceramics would be unsuccessful due to a sintering of these ceramics during the infiltration process. Another dimensional inaccuracy source is generated by the interaction between a cooling composite body and ceramic mandrel during a post-infiltration period. Discrepancy between coefficients of thermal expansion (or contraction) of a ceramic and a metal matrix composite material produces stresses within each material. Most common are compression stresses at the ceramic body and tensile stresses at the composite body, but there are also exceptions, which depend on the configuration of the ceramic mandrel and composite body. Therefore the positive supporting function of the ceramic mandrel is not valid for all volumes of a composite body. As it follows, some volumes of a tool body suffer a reduced shrinkage, but some volumes have a free shrinkage. Consequently, the '354 disclosure is not suitable as a high accuracy technique. Furthermore, the potential for cracking is also a disadvantage of the '354 disclosure because the rigid ceramic mandrel generates shrinkage stresses of the composite body. If stress therefore concentrates in definite local zones and exceeds the limit of strength, cracking is caused especially for extended zones of shrinkage deceleration.
Another disadvantage of the '354 and similar art is the tendency to fabricate a whole tool body of the same composite material, i.e. expensive powder materials and infiltrating alloys. But the specific properties of metal matrix composites for tooling applications are only needed for the tool working surfaces. It is desirable, therefore, that the tool base be made from a low cost material with high strength and without special properties like high hardness, corrosion resistance etc. These properties are useful from the point of view of stresses absorbing during the production cycle, easy machining, cost reduction, etc. Fabrication of a tool as a piece from a single metal matrix composite is not desirable, as infiltration of a large powder mass leads to increasing inaccuracy, as this process is sensitive to a scale factor. Another disadvantage of the '354 and similar patents is the low efficiency of their cooling systems.
The heat-removing function is the second main function of the injection mold, together, of course, with the shape-forming function. Therefore the need to improve heat-removal has stimulated high patent activity in the area of mold cooling. U.S. Pat. No. 5,501,592 and U.S. Pat. No. 5,656,051 disclose different designs for cooling channels with the common idea to redistribute the cooling action at all active surfaces of the mold. U.S. Pat. No. 5,207,266 offers technical solutions based on using inserts made of copper or other materials with high thermal conductivity. Nevertheless, for significant improvement of cooling ability in a mold, the teachings of '592, '051 and '266 are insufficient due to limited possibilities of disposition of the cooling channels by the conventional machining methods.
An effective solution is offered in U.S. Pat. No. 4,637,451 wherein a system of channels was replaced with a cooling chamber inside of a mold. An active surface of a mold is made as a shell with thickness of about 6-10 mm, while the internal surface of a shell is contacted with a fluid-cooling agent. The heat removing ability of this mold is extremely high due to an extensive surface of heat exchange and reduced heat resistance of the thin wall-shell. However, this design reduces the structural strength of the tool and limits the possible fields of application of this tool. Besides, the circulation of fluid cooling within the tool is not regulated and therefore the mentioned tool design does not allow one to realize a selective heat transfer, and there is a probability of over-cooling of the mold.
As it follows from the above review, the prior art has concentrated on solving separate and tedious tooling problems like cost and time reduction, increasing tool life, heat transfer improvement etc. But since all tooling problems are interrelated, solving a separate problem usually generates another problem. For example, improving tool life by employing a hard alloy creates multiple problems of machining, high cost and low constructive strength of a tool etc. That being the case, a comprehensive solution of all tooling problems remains necessary and represents yet another important goal of this invention as set out below.